Atomic absorption spectroscopy is a method for determining the concentration of a looked-for element in a sample which is to be analyzed. By an atomizing device, e.g., a burner with a flame, the sample which is to be analyzed is "atomized". Then, the components of the sample are present in an atomic state. A measuring light beam is passed through such an atomized sample. The measuring light beam is generated by a line emitting light source, e.g., a hollow cathode lamp. The spectral lines of the measuring light beam correspond to the resonant lines of the looked-for element. Therefore, the absorption to which the measuring light beam is subjected in the atomized sample depends on the number of atoms of the looked-for element and thus on the concentration of this element in the sample. The measuring light beam impinges on a detector which provides a corresponding signal. By means of a calibration sample this signal can be calibrated such that it provides the concentration of the sample.
The concentration of the looked-for element in the sample has to be within a certain measuring range. When the concentration is too small, the signal at the detector becomes too small and disappears in the noise. When the concentration becomes too large, the measuring light beam is entirely absorbed such that measurements are also impossible. Between these two points lies the measuring range wherein the signal of the detector depends in a substantially linear manner on the concentration of the looked-for element. Therefore, if required, the sample has to be diluted such that the concentration of the looked-for element is within this optimal measuring range.
It is known to supply samples to an atomizing device of an atomic absorption spectrometer by "flow injection". The sample is introduced into a loop of tubing. The atomizing device is supplied by a continuous carrier liquid flow. By means of a change-over valve (injection valve) the loop is connected into the carrier liquid flow. In this way the sample is supplied by the carrier liquid flow from the loop of tubing to the atomizing device. The sample forms a plug in the carrier liquid flow. On its way to the atomizing device, this plug diffuses to an approximately bell-shaped distribution of the concentration of sample liquid in carrier liquid.
Accordingly, the atomic absorption spectrometer provides a transient signal in the shape of an approximately bell-shaped peak. The shape of this peak is the same for all concentrations of the looked-for element within the linear measuring range. The peaks differ only by the ordinate which is proportional to the concentration.
By the publication of Olsen, Ruzicka and Hansen "Gradient Techniques In Flow Injection Analysis" in "Analytical Chimica Acta" 136 (1982) 101-112, particularly FIGS. 6 and 7 and the text referring thereto on page 108-109, a method for generating a family of calibration curves with different dilutions of the sample is known. Several calibration curves are measured in which the looked-for element is present in the different concentrations. Signal peaks result which have substantially the same waveform but different amplitudes. The signals are scanned at their descending slope of the signal peaks at different scanning times, each referred to the time of the signal maximum. Each of these scanning times provides a calibration curve which corresponds to another dilution of the sample. The signal of an unknown sample is scanned at such scanning time at which the amplitude of its signal peak is in the optimal, linear measuring range. Thus, the dilution of the sample is superseded by the selection of the scanning time. This is called "electronic dilution". This "electronic dilution" is also described in "Fresenius Zeitschrift fur Analytische Chemie" (1988, 329:678-684).
By a publication of B. V. L'vov "Graphite Furnace Atomic Absorption Spectrometry on the Way To Absolute Analysis" in "J. Anal. At. Spectrom.", 3 (1988, 9-12), a method is known for determining the concentration of an element looked-for in a sample by means of atomic absorption spectroscopy in which the sample is electrothermally atomized. A sample is introduced into a graphite furnace. The graphite furnace is heated to a high temperature. Thereby, a "cloud of atoms" is generated within the graphite tube in which cloud the components of the sample are present in an atomic state. The measuring light beam is passed through the graphite tube. Here also, a bell-shaped, transient signal results. The sample is atomized whereby the signal increases and, thereafter, the atomic vapor is removed from the graphite furnace by an inert gas flow whereby the signal again decreases.
In the method described by L'vov two calibration solutions are used. An initial calibration solution comprises the looked-for element in a relatively low concentration although sufficiently far beyond the detection limit. This calibration solution generates a signal within the linear range of the atomic absorption spectrometer. The second calibration solution comprises the looked-for element in a concentration which is at the upper end of the detection range. The signal course obtained with the second calibration solution is compared point for point with the corresponding signal obtained with the initial calibration solution. If the atomic absorption spectrometer were to operate linearly throughout the entire detection range, the signals would behave at any time as the concentrations of the two calibration solutions. Using the divergency of the signal actually measured with the second calibration solution from the value calculated with a linear calibration curve taken as a basis, a calibration curve can be determined by regression calculation. By means of this calibration curve the signal course of an unknown sample can be linearized. From the linearized signal course either the peak level or the peak area can be determined for the signal evaluation.